The issue of disposability of products is of great concern to the nonwovens industry. Among the different types of disposable nonwovens are pre-moistened nonwovens which are readily dispersible in large amounts of water or liquids, and absorbent, disposable nonwovens that can be used as wipes and require a high level of strength after contact with liquids.
In recent years a large number of wipes and wiper products have been introduced which claim that they can be flushed after use. Most of these products are flushable based on size; i.e., the wipe or wiper is small enough to pass through the opening at the bottom of the toilet and pass through the plumbing and sewer lines under ideal conditions. These products are generally based on hydroentangled nonwoven webs which do not break up in the flushing process. These products can compromise sewer or drain lines in several ways. If a larger than recommended number of wipes are flushed (for example, by a child) these wipes can get stuck in pipe elbows, create a plug which prevents water from passing and creating a back up. Also these wipes can get caught on tree roots which sometimes protrude into sewer lines creating plugs and back ups. It is highly desirable for these nonwoven wipes or wipers to disperse allowing the webs to break up into individual cellulose fibers which can more easily pass the obstacles encountered in the sewer and drain lines.
With regard to dispersible nonwovens, the products may be produced as dry material, such as tissues or wipes, which will be truly flushable and dispersible when deposited in a large amount of water or aqueous solution. The tissues or wipes must disintegrate in water under gentle agitation without the addition of temperature or chemicals.
Water soluble or redispersible polymeric binders have been used in making nonwoven substrates and they are generally comprised of poly(vinyl alcohol) (PVOH), PVOH-stabilized vinyl acetate or vinyl acetate based polymers, e.g., vinyl acetate-ethylene (VAE) emulsion polymers. The treated dry nonwoven webs needs to have enough wet strength so that they will not disintegrate during use but will disintegrate in the presence of excess water such as the water in a toilet. Many current commercial nonwoven webs have either no wet strength or have a lot of permanent wet strength. The former are unsuitable for use in products such as feminine hygiene products because they fall apart on insult. The latter cannot be safely discarded in toilets without the risk of clogging pipes.
Several methods have been proposed to address disintegration of a dry nonwoven product during use. One method employs the addition of boric acid or derivatives to the aqueous lotion composition which is applied to a nonwoven substrate in order to maintain the integrity of the nonwoven substrate. The following patents illustrate the prior art of this technology:
U.S. Pat. No. 4,245,744 discloses nonwoven fiber sheets impregnated with PVOH-containing vinyl acetate-based polymers in which the nonwoven sheets are maintained in contact with a dilute aqueous solution of a precipitating or gelling agent for PVOH, such as boric acid.
U.S. Pat. No. 5,252,332 discloses a packaged towelette comprising a sheet of nonwoven fibers impregnated with PVOH or a PVOH-containing binder in contact with an aqueous solution containing boric ions and bicarbonate ions.
U.S. Pat. No. 5,629,081 discloses a pre-moistened, dispersible and biodegradable wet wipe comprising a web of nonwoven fibers contacted with a PVOH-containing binder. The binder-contacted web further comprises an aqueous lotion solution comprising 0.1-0.9 wt % boric acid and 5-8 wt % alkali metal bicarbonate, based on weight of the lotion.
U.S. Pat. No. 5,384,189 discloses a water decomposable nonwoven fabric in which the fibers are bonded to one another with a water-soluble binder comprising an unsaturated carboxylic acid/unsaturated carboxylic acid ester copolymer in which 1-60 mole % of the repeating units derived from the carboxylic acid is in the form of a salt. The binder is soluble in tap water but is insoluble in an aqueous solution containing not less than 0.5 wt % of a neutral inorganic salt comprising a monovalent ion.
U.S. Pat. No. 5,972,805 discloses a water-soluble polymeric binder composition for use in making nonwoven webs comprising 25-90 wt % unsaturated carboxylic acid/unsaturated carboxylic acid esters/ester copolymer, 10-75 wt % divalent ion inhibitor and 0-10 wt % plasticizer. The water-soluble binder composition is soluble in an aqueous environment having a divalent ion concentration of less than about 50 ppm and a monovalent concentration of less than about 0.5 wt %.
U.S. Pat. No. 5,256,417 discloses a packaged towelette which is disposable and comprises a sheet of nonwoven fibers impregnated with a binder, e.g., PVOH or an aqueous polymer emulsion containing PVOH as a protective colloid. The sheet is maintained in a wet condition within the package by contact with a non-aqueous lotion composition which is a liquid organic compound that is a non-solvent for PVOH.
U.S. Pat. Nos. 5,976,694 , 6,121,170, and 6,495,080 disclose disposable products which are flushable. A feature of the flushable product is that it has sufficient wet strength for its intended use but loses its structural integrity upon contact with large amounts of water. The compositions employed in forming the disposable product comprise a blend of at least one water-sensitive polymer and at least one polymer selected from polylactide, polyolefin-grafted with a polar group, e.g., maleic acid, and other aliphatic polyesters. Dispersibility depends upon the amount of monovalent or multivalent ions in the aqueous solution.
WO 00/39378 discloses a water-dispersible nonwoven fabric containing a temperature-sensitive or ion-sensitive binder material, which is useful in the manufacture of flushable personal care products.
Additives such as boric acid and mono- or multi-valent ions are typically called triggers in the industry. These triggers interact with water soluble binders to increase wet tensile strength of the web. This allows the webs, bound with a combination of water soluble binder and trigger, to function in applications such as wet wipes, where the web needs to maintain its integrity under conditions of use. When the web is placed in excess water (i.e., in a toilet bowl) the concentration of these triggers is diluted, breaking up the interaction between the binder and trigger and causing a loss of wet tensile strength. When the wet tensile strength of the web is diminished, it can break up under agitation and separate into smaller pieces which can more easily pass through the drain and sewer lines.
For pre-moistened products, airlaid webs can be manufactured with water soluble binders and the trigger can be added to the lotion creating the interaction.
There is a need in the industry for improved and economical methods of incorporating triggers into dry nonwoven products. The method should result in a product which maintains its integrity when made wet or damp, but enable complete dispersion in large amounts of water or aqueous liquids such as is found in toilets.